Filenames

Filenames are the human-readable identifiers assigned to files within a filesystem. A filename, together with its path, uniquely identifies a file within a directory. In many systems the filename includes an extension that indicates the file type or associated program, though extensions are not required and may be hidden by settings.

Operating systems impose rules about allowed characters and separators. Windows forbids characters such as < > : " / \ | ? * and reserves certain names (CON, PRN, AUX, NUL, COM1-9, LPT1-9). Filenames are typically case-insensitive on Windows, and the usual maximum length for a name is 255 characters, with full path length historically limited to 260 characters; recent versions offer longer paths with configuration. Unix-like systems allow a wider range of characters in filenames, with the path separator being / and the NUL character forbidden; filenames are case-sensitive. Filenames are stored as bytes, and compatibility with Unicode depends on the filesystem and software in use (UTF-8 is common in modern systems).

Encoding and normalization considerations affect portability. Most modern systems support Unicode for filenames, but behavior can vary by filesystem and locale. To improve cross-platform compatibility, users often prefer ASCII-compatible names and avoid characters that are problematic in some environments.

Best practices include using descriptive, portable names, avoiding spaces where possible and preferring hyphens or underscores. Keep extensions clear and, on platforms where it matters, use lowercase extensions. Do not rely on extensions for security or behavior, and be mindful of platform-specific reserved names and length limits when sharing files across systems.